Control Key :
A user-defined key that determines how any operation or sub-operation is handled in order processing and in product costing.
You have to assign control key in Routing.
eg.
PP01-Control key doe not have Auto GR active in SAP Standrad, so you have to do GR using MIGO or MB1C using movement type 101
Operation Control Key -PP03 in standard has Auto GR active and it can be used only in the Last Operation in any production order if you want to avoid manual MIGO-101 or want to have Auto Goods Recipt .
for more information refer to the link.
http://help.sap.com/saphelp_crm60/helpdata/en/84/9d6d86d5d311d1950200a0c93033d8/content.htm